During centrifugation, in particular in very fast turning lab centrifuges heat is generated during rotation of the centrifuge rotor in the centrifuge bowl through air friction and introduction of dissipated electrical power. Since the centrifuge bowl is closed with a lid in order to prevent centrifuged material from exiting the introduced heat cannot be easily dissipated and eventually causes an increase in temperature in the material that is being centrifuged.
The temperature increase is undesirable since it can lead to destruction or uselessness of the centrifuged samples. Typically the samples have to be kept at a defined temperature, for example depending on the application at a temperature of 4° C., 22° C., or 37° C. Therefore measures were already taken in the past in order to prevent an increase of a temperature of the centrifuged material, wherein indirect cooling is typically used. For this indirect cooling the rotor is typically enclosed in the centrifuge bowl under the centrifuge cover and no cooling channel or similar is provided. Air therefore only circulates within the centrifuge bowl. Cooling is only provided through a second medium which is run along an outside of the bowl or in a wall of the bowl. Thus, typically a compressor cooling device with tubes and heat exchangers is provided through which a special refrigerant (which differs from coolants as they are run for example in cooling water cycles of cars, a refrigerant goes through phase changes when going through the refrigeration cycle, typically from liquid to gaseous and this refrigerant also facilitates temperature controlling a material to be cooled to a temperature that is below ambient temperature) is run through conduits forming the refrigerant cycle which contact the centrifuge bowl for example in spirals, this means the side wall and the base of the bowl, and run along the bowl in order to dissipate heat. A compressor cooling device of this type also facilitates cooling the sample material to a temperature below a temperature of ambient air.
Compressor cooling devices 1 include an evaporator 3 which is typically run as a conduit about the centrifuge bowl 5, a compressor 7, a condenser 9 and an expansion element 11 (c.f. FIG. 1). Thus, the expansion element 11 is configured for the highest load case, thus the maximum speed of the centrifuge rotor (not illustrated) wherein it is already known that the expansion element (which is a pressure balancing element between a high pressure side and a low pressure side of the refrigerant cycle when the compressor is stopped) is configured as a capillary tube or a thermostat injection valve 11.
In combination with pressure controlled temperature detection 13 after the evaporator 3 the thermostat controlled injection valve (TEV) 11 is used for automatically increasing or throttling a refrigerant in flow in the refrigeration cycle 15 at the evaporator inlet VE as a function of the determined temperature. Thus, super heating the refrigerant at an evaporator outlet VA is required so that a positive pressure is generated which is directly conducted onto a spring 17 of the thermostat controlled injection valve 11 in order to actuate the injection valve. Put more precisely a particular temperature is provided at the evaporator outlet VA. The sensor 13 of the TEV 11 is attached at the evaporator outlet VA, wherein refrigerant is provided at the evaporator outlet. Based on the temperature at the evaporator outlet VA the refrigerant has a respective pressure which then impacts the TEV 11 and counteracts the reset force of the spring so that the TEV 11 opens or closes.
An additional control element, for example a frequency controlled compressor 7 facilitates partially but imprecisely controlling other load cases.
Since over heating the refrigerant is required in order for the thermostat controlled injection valve 11 to function the evaporator performance cannot be used in its entirety, only approximately 95% of the evaporator surface can be used. Due to the required superheating a temperature differential of approximately 7K is provided between the evaporator inlet VE and the evaporator outlet VA.
Another essential disadvantage of such known compressor cooling devices 1 in centrifuge is that the compressors 7 can only be power controlled rather imprecisely and within certain limits, so that the compressor 7 may have to be switched off completely in various partial load cases and also low load cases.
This, however, is not possible all the time because the compressors 7 typically have a minimum run time in order to assure internal oil circulation. Vice versa due to the increased heating of the drive motor of the compressor 7 during start up and the required pressure balancing or pressure differential reduction between high pressure side and low pressure side a certain minimum shut down time is provided for such compressors 7. Therefore controllability through the compressor 7 is severely limited in particular in the low power range.
An additional disadvantage is that vibrations are generated during start up or stopping of the compressor 7 of a compressor cooling device 1. The vibrations influence operating parameters of the centrifuge, increase the remix rate in the rotor after stopping the centrifuge and impact lab equipment and similar arranged proximal to the centrifuge. Last not least, frequently turning the compressor 7 on and off reduces its service life.